1. Technical Field of the Invention
The present invention relates to a mounting arrangement for a lens or a lens assembly which permits calibration of the lens focus during assembly of an image sensor. The invention is particularly, but not exclusively, applicable to the manufacture of small, inexpensive solid state image sensors for use in products such as cameras, mobile telephones, webcams or medical devices. The invention has particular, but not exclusive, application to autofocus lens mounting assemblies for such devices.
2. Description of Related Art
It is known to provide an image sensor assembly comprising a solid state single chip image sensor and a lens, wherein the position of the lens can be adjusted with respect to the image sensor during manufacture to achieve optimal focus of the image sensor on the surface (i.e. to permit calibration of the image sensor assembly). Typically the lens is mounted in a lens barrel which has a screw threaded engagement with a chip mount. Calibration of the focus of the lens can then be achieved by relative rotation of the lens barrel within the chip mount.
Typically it is necessary to move the lens barrel with respect to the image sensor through increments of as little as 5 μm to achieve the required focus adjustment. This can be achieved by proving a thread having a pitch of 0.35 mm whereby 5° rotation of the lens barrel with respect to the chip holder provides an axial adjustment of the lens of 5 μm.
The lens barrel must subsequently be locked relative to the chip holder, for example by bonding with an adhesive. However, the calibration of the lens focus is achieved in an automated process in a clean room environment that is not tolerant to the presence of adhesive. Therefore it is typically necessary to apply adhesive to lock the lens barrel in position in a separate step at a separate location from the focus calibration step. There is a risk that the lens barrel may move relative to the chip holder during transfer of the image sensor assembly between said steps, resulting in incorrect calibration of the lens.
One known way to avoid this problem is to provide a tight interference fit between at least a portion of the lens barrel and the chip holder. However, this leads to the risk of small particles being dislodged from the cooperating surfaces of the lens barrel and the chip holder leading to contamination of the lens and unacceptable image quality. Furthermore, in the case of autofocus lenses, the lens barrel is mounted within an actuator carriage assembly which is in turn adjustably mounted on the chip holder. Size limitations limit the possible wall thickness of the actuator carriage assembly such that the assembly is not tolerant of such an interference fit between the lens barrel and the actuator carriage assembly without risking damage to the actuator carriage assembly.